In both the NTSC and PAL color television systems the luminance information is conveyed in a wide band and the color information is conveyed by modulation of a subcarrier that is fairly high up in the band. In order to separate the luminance signal from the color signal a notch filter is used to exclude the color subcarrier and its sidebands, but in order to avoid ringing the notch filter starts reducing the amplitude of the luminance signal at frequencies as low as 2.0 MgHz so that much of the luminance detail is lost.
In order to increase the amplitude of the higher frequency luminance components without contaminating them with the subcarrier and its sidebands, comb filters have been used. They basically operate on the principle that the luminance signals of two scanning lines can be added together if the color subcarrier components on the two lines are out of phase so as to cancel. In the NTSC system, the subcarrier is out of phase on adjacent lines so that what is termed a one line comb filter is used to delay one line by a line scanning interval before that line is added to the next. In the PAL system the color subcarrier is out of phase at two line intervals so that a two line comb filter must be used.
In both cases, however, any difference in phase or amplitude of the subcarrier components or any vertical change in luminance occurring in the lines being added will prevent perfect cancellation and produce artifacts that may make the picture worse. Circuits have been provided that remove the comb filter and permit notch filter operation when this condition becomes too severe. Thus, you either have comb filter operation or notch filter operation.